Because the 30% is "opt-in" not automatic, the $1m threshold applies to all your apps and associated accounts apps combined, and $1m is just not a big target anymore considering 8400 people on $10/month subscriptions will get you over that line.

And of course, when they announced this big discount it was reported to apply to about 5% of actual IAP spending, which would make the average commission fee being paid around 29%.

tldr; it's a weaselly discount.